It's going to be hard pinpointing an external display with white casing that matches the quality and look of the panel inside the iMac. I've got a 19" Polyview hooked up to mine, and while the casing or resolution don't necessarily match, both tend to go away when you actually start working on something. I would just get the 20" and be done with it, that way it will match any Apple headless you might decide to get after your iMac.